Covering extracellular polymeric substances to enhance the reactivity of sulfidated nanoscale zerovalent iron toward Cr(VI) removal

For effective removal and recovery of carcinogenic hexavalent chromium (Cr(VI)), we synthesized sulfidated nanoscale zerovalent iron@extracellular polymeric substances (SnZVI@EPS) with the aim of combining the respective advantages of EPS and SnZVI. EPS was introduced during the synthesis of SnZVI and successfully covered SnZVI, resulting in a larger specific surface area and better environmental friendliness of SnZVI@EPS over that of SnZVI (108.6362 vs. 10.9331 m(2)/g, 4.04-log vs. 6.42-log Escherichia coli reduction). SnZVI@EPS was more effective than SnZVI in removing Cr(VI) (64.90 mg/g vs. 43.76 mg/g), also under anaerobic, aerobic, wide pH, and high concentration of coexisting ions. The total Cr removal by SnZVI@EPS was higher than that of SnZVI (64.64 mg/g vs. 41.83 mg/g). The pseudo-second order model described well the adsorption process of SnZVI and SnZVI@EPS with equilibrium removal capacity of 44.21 and 66.18 mg/g for 20 mg/L Cr(VI) removal. Chemisorption and monolayer-based adsorption were dominated during Cr(VI) adsorption by SnZVI@EPS. The covered EPS provided better Cr(VI) removal and Cr(III) precipitation of SnZVI@EPS than SnZVI through the faster electron transfer, stronger solid-liquid reactions between SnZVI@EPS and Cr(VI), and additional binding sites for Cr(III)-Fe(III) coprecipitation. This work proved that environmental friendliness and efficient Cr(VI) removal and recovery of SnZVI can be improved by EPS modification.